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We employed a Bayesian model to assess the metabolic state of 8 Estonian lakes representing the 8 lake types according to the European Union Water Framework Directive. We hypothesized that long-term averages of light-related variables would be better predictors of lake metabolism than nutrient-related variables. Model input parameters were in situ high-frequency measurements of dissolved oxygen, temperature, and irradiance. Model simulations were conducted for several (5–12) diel cycles for each lake during the summer season. Accounting for uncertainty, the results from the Bayesian model revealed that 2 lakes were autotrophic for the duration of the experiment, 1 was heterotrophic, and 5 were balanced or had an ambiguous metabolic state. Cross-comparison with a traditional bookkeeping model showed that the majority of lakes were in metabolic balance. A strong coupling between primary production and respiration was observed, with the share of autochthonous primary production respired by consumers increasing with light extinction and nutrient-related variables. Unlike gross primary production, community respiration was strongly related to light extinction, dissolved organic carbon (DOC) and total phosphorus. These findings suggest that a drastic decrease in light-limited primary production along the DOC gradient counter-balanced nutrient supply in the darker lakes and thus blurred the relationship between primary production and nutrients. Thus, contrary to our hypothesis, both light and nutrient-related variables seemed to be good predictors of lake respiration and its coupling to lake primary production.
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... In our case, daily metabolic estimates showed large variation, particularly during the summer months, that reflects not only the inherent uncertainty of metabolism estimation but also the dynamic behavior of shallow lake processes. Although large day-to-day variation is often attributed to methodological noise and uncertainty [14,37], there are also studies that underline the role of extreme climate events (storms and floods) in causing irregularities in metabolic dynamics [9]. Additionally, it was shown by Hanson et al. [38] that internal waves and other short-term mixing events (e.g., diurnal mixing phenomena) can affect the atmospheric flux of dissolved oxygen altering the metabolic estimates. ...
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... Rapid development of in situ sensors and platforms allows higher frequency measurements of fluorescence, turbidity, oxygen, and other parameters on moorings or even on autonomous underwater vehicles (AUVs), creating 3D scans of lake parameters (Cremona et al. 2016, Meinson et al. 2016. Still, some measurements that require lab work will remain difficult to carry out at high frequency, such as measuring phytoplankton primary production (PP). ...
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... Higher temperatures increase carbonate weathering rates and hence in-lake alkalinity, pH, and dissolved inorganic carbon concentration, all of which enhance photosynthesis of cyanobacteria (Paerl et al. 2001). Very alkaline waters lead to CO 2 supersaturation which reduces the limitation of primary production that might occur after strong algal growth Nõges, Järvalt et al., 2016b, Cremona et al. 2016b. ...
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PLANKTONIC bacteria are a fundamental component of the organic carbon cycle in aquatic systems1. Organic carbon consumption by planktonic bacteria is the sum of bacterial production (BP) and bacterial respiration (BR). It is now estimated that 30-60% of phytoplankton production (the amount of inorganic carbon fixed by phytoplankton photosynthesis, corrected for phytoplankton respiration) in marine and freshwater systems is processed by bacteria1-3. These estimates of carbon flow through bacteria are conservative, however, because losses due to bacterial respiration are seldom directly measured4,5. We report here that bacterial respiration is generally high, and tends to exceed phytoplankton net production in unproductive systems (less than 70 to 120 μg carbon per litre per day). A large proportion of the world's aquatic systems have phytoplankton productivities below this value6. Bacterial growth efficiency (BGE) is the result of BP and BR[BGE = BP/(BR + BP)]. Comparisons of our models of bacterial respiration with published models of bacterial secondary production1,7 show that bacterial growth efficiency must range from less than 10% to 25% in most freshwater and marine systems, well below the values commonly assumed in many current ecological models1,2,8,9. The imbalance between bacterial respiration and phytoplankton production suggests that in unproductive aquatic systems, the biological system is a net source of CO2.
Article
Gross primary production (GPP) and community respiration (R) are increasingly calculated from high-frequency measurements of dissolved oxygen (DO) by fitting dynamic metabolic models to the observed DO time series. Because different combinations of metabolic components result in nearly the same DO time series, theoretical problems burden this inverse modeling approach. Bayesian parameter inference could improve identification of processes by including independent knowledge in the estimation procedure. This method, however, requires model development because parameters of existing metabolic models are too abstract to achieve a significant improvement. Because algal biomass is a key determinant of GPP and R, and high-frequency data on phytoplankton biomass are increasingly available, coupling DO and biomass time series within a Bayesian framework has a high potential to support identification of individual metabolic components. We demonstrate this potential in 3 lakes. Phytoplankton data were simulated via a sequential Bayesian learning procedure coupled with an error model that accounted for systematic errors caused by structural deficiencies of the metabolic model. This method provided ecologically coherent, and therefore presumably robust, estimates for biomass-specific metabolic rates and contributes to a better understanding of metabolic responses to natural and anthropogenic disturbances.
Article
Over the past 15 years, an increasing number of studies in limnology have been using data from high-frequency measurements (HFM).This new technology offers scientists a chance to investigate lakes at time scales that were not possible earlier and in places where regular sampling would be complicated or even dangerous. This has allowed capturing the effects of episodic or extreme events, such as typhoons on lakes. In the present paper we review the various fields of limnology such as monitoring, studying highly dynamic processes, lake metabolism studies, and budget calculations where HFM has been applied, and which have benefitted most from the application. Our meta-analysis showed that more than half of the high-frequency studies from lakes were made in North-America and Europe. The main field of application has been lake ecology (monitoring, lake metabolism) followed by physical limnology. Water temperature and dissolved oxygen have been the most universal and commonly measured parameters and we review the various study purposes for which these measurements have been used. Although a considerable challenge forthe future, our review highlights that broadening the spatial scale of HFM would substantially broaden the applicability of these data across a spectrum of different fields.
Article
A ten-year (1995-2005) research programme SUVI (Suomi-Viro) on the optics of Finnish and Estonian lakes has been completed. The objectives were to examine the light conditions, to map optically active substances (OAS), and to develop remote sensing methods. Altogether 14 Estonian and 7 Finnish lakes representing different types of water, from oligotrophic to hypertrophic and dystrophic were included. We performed extensive analyses of concentrations of three main OAS, light attenuation and Secchi depth. They varied among lakes and with seasons, but no systematic temporal change could be detected during the 10-year period. We studied the underwater light field using a spectrometer and two PAR quantum sensors, and elaborated three versions of optical classification of lake waters based on: (1) apparent optical properties and the amount of OAS, (2) irradiance reflectance spectra, and (3) light attenuation coefficient and the predominant OAS. We developed two models for determining the diffuse attenuation spectra of light, using data for (1) one wavelength and (2) three wavelengths, and elaborated a semiempirical model for quantitative description of the underwater light regime. We developed a bio-optical model for interpretation of remote sensing data that allows simulation of the reflectance spectra based on the concentrations of OAS. Winter expeditions were undertaken to examine the optical properties of ice and snow and the light conditions in the water beneath the ice cover. Gas pockets are the main optically active impurity in snow and ice. The transparency of ice is similar to that of lake water. The ice cover lowers the light level and makes the light more diffuse in the near surface liquid water layer.
Chapter
Respiration represents the major area of ignorance in our understanding of the global carbon cycle. In spite of its obvious ecological and biogeochemical importance, most oceanographic and limnological textbooks deal with respiration only superficially and as an extension of production and other processes. The objective of this book is to fill this gap and to provide a comprehensive review of respiration in the major aquatic systems of the biosphere. The introductory chapters review the general importance of respiration in aquatic systems, and deal with respiration within four key biological components of aquatic systems: bacteria, algae, heterotrophic protists, and zooplankton. The central chapters of the book review respiration in major aquatic ecosystems: freshwater wetlands, lakes and rivers, estuaries, coastal and open oceans, and pelagic ecosystems, as well as respiration in suboxic environments. For each major ecosystem, the corresponding chapter provides a synthesis of methods used to assess respiration, outlines the existing information and data on respiration, discusses its regulation and links to biotic and abiotic factors, and provides regional and global estimates of the magnitude of respiration. This is followed by a chapter on the modelling of respiration for various components of the plankton. The final chapter provides a general synthesis of the information and data provided throughout the book, and places aquatic respiration within the context of the global carbon budget.
Article
Dissolved organic carbon (DOC) concentrations in lakes are changing globally, but little is known about potential ecosystem impacts.We evaluated the relationship between DOC and whole-lake primary production in arctic and boreal lakes. Both light extinction (inhibits primary production) and nutrient availability (stimulates primary production) are positively and nonlinearly related to DOC concentration. These nonlinearities create a threshold DOC concentration (4.8 mg L−1), below which the DOC-primary production relationship is positive, and above which the relationship is negative. DOC concentration varies maximally between regions, creating a unimodal relationship between primary production and DOC that emerges at broader scales because arctic lakes largely fall below the threshold DOC concentration, but boreal lakes fall above it. Our analysis suggests that the impact of DOC trends on lake primary production will vary across lakes and regions as a result of contrasting baseline conditions relative to the DOC threshold.
Article
We evaluated several potential drivers of primary production by benthic algae (periphyton) in north-temperate lakes. We used continuous dissolved oxygen measurements from in situ benthic chambers to quantify primary production by periphyton at multiple depths across 11 lakes encompassing a broad range of dissolved organic carbon (DOC) and total phosphorous (TP) concentrations. Light-use efficiency (primary production per unit incident light) was inversely related to average light availability (% of surface light) in 7 of the 11 study lakes, indicating that benthic algal assemblages exhibit photoadaptation, likely through physiological or compositional changes. DOC alone explained 86% of the variability in log-transformed whole-lake benthic production rates. TP was not an important driver of benthic production via its effects on nutrient and light availability. This result is contrary to studies in other systems, but may be common in relatively pristine north-temperate lakes. Our simple empirical model may allow for the prediction of whole-lake benthic primary production from easily obtained measurements of DOC concentration.
Article
We investigated the influence of light, nutrients, and organic matter on gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP = 5 GPP - R) in a dystrophic forest lake and an open eutrophic lake. Forest vegetation reduced incoming irradiance (20%) and wind speed (34%) in dystrophic Gribso, having thermal stratification 1 month longer than in eutrophic Slotsso. While Gribso had nutrient-limited phytoplankton during most of the year, Slotsso only experienced nutrient depletion during algal blooms. Colored dissolved organic matter (CDOM) absorbed most light (average 82%) in dystrophic Gribso, while phytoplankton and other particles absorbed most light (45%) in eutrophic Slotsso. GPP and NEP were positively related to irradiance in both lakes. However, because of higher CDOM absorbance, three times more light was needed to attain autotrophy in Gribso, being net heterotrophic (NEP < 0) for 79% of all days, compared to 59% in Slotsso. This difference vanished when NEP was scaled to light absorption by pigments, although the eutrophic lake maintained a higher photon yield. Metabolic rates varied much more in Slotsso, where higher light and nutrient availability facilitated occasional phytoplankton blooms, while low light and nutrient availability in Gribso dampened temporal variability. Both lakes were annually net heterotrophic with similar annual areal rates (NEP, -14 mol C m(-2)). Net heterotrophy in dystrophic Gribso derives from high import of organic carbon-rich water, while heterotrophy in eutrophic Slotsso is fueled by degradation of sediment pools of organic matter accumulated under previous hypereutrophic conditions, emphasizing the importance of lake history on the contemporary metabolic state.
Article
The diffusion coefficients D of important gas tracers dissolved in water an seawater were measured with a modified Barrer method. The measurements include the gases He, Ne, Kr, Xe, H2 , CH4 , and CO2 dissolved in distilled water in the temperature range from 5 to 35°C, and He and H2 dissolved in seawater in the same temperature range. The maximum systematic error is estimated to be well below 5%. The isotopic fractionation in the diffusion coefficient, εD, was determined to be (-0.87+/-0.05)%o for 13 CO2 /12 CO2 and (15+/-3)% for 3 He/4 He.
Article
In 12 lakes a significant fraction of the hypolimnetic oxygen deficit was produced by the respiration of seston in the hypolimnetic water column. Mean summer seston respiration rates ranged between 4 and 80 mg O2∙m−3∙d−1. Rates of seston respiration were proportional to the in situ water temperature and to the concentration of Chlorophyll a. The amount of oxygen consumed in the water column and the fraction of the total oxygen deficit produced by sestonic respiration were correlated with the amount of phosphorus sedimented from the epilimnion. Fifteen to 66% of the total oxygen consumption occurred in the water column of the hypolimnion. Seston respiration was a larger proportion of the total respiration in the hypolimnion of lakes with a thick hypolimnion than in lakes with a shallow hypolimnetic water column.
Article
We assembled data from a global network of automated lake observatories to test hypotheses regarding the drivers of ecosystem metabolism. We estimated daily rates of respiration and gross primary production (GPP) for up to a full year in each lake, via maximum likelihood fits of a free-water metabolism model to continuous high-frequency measurements of dissolved oxygen concentrations. Uncertainties were determined by a bootstrap analysis, allowing lake-days with poorly constrained rate estimates to be down-weighted in subsequent analyses. GPP and respiration varied considerably among lakes and at seasonal and daily timescales. Mean annual GPP and respiration ranged from 0.1 to 5.0 mg O2 L−1 d−1 and were positively related to total phosphorus but not dissolved organic carbon concentration. Within lakes, significant day-to-day differences in respiration were common despite large uncertainties in estimated rates on some lake-days. Daily variation in GPP explained 5% to 85% of the daily variation in respiration after temperature correction. Respiration was tightly coupled to GPP at a daily scale in oligotrophic and dystrophic lakes, and more weakly coupled in mesotrophic and eutrophic lakes. Background respiration ranged from 0.017 to 2.1 mg O2 L−1 d−1 and was positively related to indicators of recalcitrant allochthonous and autochthonous organic matter loads, but was not clearly related to an indicator of the quality of allochthonous organic matter inputs.
Article
A precise oxygen method was used to measure primary production, community respiration and to determine the importance of exogenous organic carbon as an energy source to planktonic communities in the epilimnion of 12 oligotrophic to mesotrophic Shield lakes. Median photosynthetic parameters observed with the oxygen method were up to twice as high as those measured with 14C in other oligotrophic Shield lakes. Gross photosynthesis was almost always larger than community respiration, with a median P: R ratio of 1.7. We observed strong relationships between respiration and gross photosynthesis, but could not detect any significant trend between respiration or the P: R ratio and the concentration of dissolved organic carbon (DOC). DOC appeared to depress both photosynthesis and respiration. These results argue against the importance of exogenous organic carbon supply as a significant energy source to freshwater planktonic communities. Previously low-P:R ratios reported for oligotrophic fresh waters may be due to the uncertain meaning of 14C production data.
Article
Respiratory CO 2 release from inland waters is a major process in the global carbon cycle, retaining more than half of the carbon flux from terrestrial sources that otherwise would reach the sea. The strongly lake type-specific balance between primary production and respiration determines whether a lake acts regionally as a net sink or source of CO 2 . This study presents two-year (2009, 2010) results of high-frequency metabolism measurements in the large and shallow polymictic eutrophic Lake Võrtsjärv (area 270 km 2 ; mean depth 2.8 m). We estimated the net ecosystem production (NEP), com-munity respiration (R) and gross primary production (GPP) from continuous measurements of oxygen, irradiance, wind and water temperature. A sinusoidal model fitted to the calculated metabolic rates showed the prevalence of net autotrophy (mean GPP:R [ 1) from early spring until August/September, whereas during the rest of the year heterotrophy (mean GPP:R \\ 1) prevailed, characterizing the lake as CO 2 neutral on an annual basis. Community respira-tion lagged behind GPP by approximately 2 weeks, which could be explained by the bulk of the phyto-plankton biomass accounted for by filamentous cya-nobacteria that are considered mostly inedible to zooplankton, and the seasonally increasing role of sediment resuspension. In the warmer year 2010, the seasonal peaks of GPP, R and NEP were synchro-nously shifted nearly 1 month earlier compared with 2009. The strong stimulating effect of temperature on both GPP and R and its negative effect on NEP revealed by the multiple regression analysis suggests increasing metabolic rates and increasing heterotro-phy in this lake type in a warmer climate.
Article
Recent literature has suggested that for many lakes and rivers, the respiratory breakdown of organic matter (R) exceeds production of organic matter by photosynthesis (gross primary production [GPP]) within the water body. This metabolic balance (GPP < R; "heterotrophy") implies that allochthonous organic matter supports a portion of the aquatic ecosystem's respiration. Evidence that many lakes are heterotrophic comes from diverse approaches, and debate remains over the circumstances in which heterotrophy exists. The methods used to estimate GPP and R and the limited extent of lake types studied, especially with respect to dissolved organic carbon (DOC) and total phosphorus (TP) concentrations, are two reasons for differing conclusions. We deployed O2 and CO2 sondes to measure diel gas dynamics in the surface waters of 25 lakes. From these data, we calculated GPP, R, and net ecosystem production (NEP = GPP - R). Over the broad range in TP and DOC among the lakes, diel CO2 and O2 changed on a near 1:1 molar ratio. Metabolism estimates from the two gases were comparable, except at high pH. Most lakes in our data set had negative NEP, but GPP and R appeared to be controlled by different factors. TP correlated strongly with GPP, whereas DOC correlated with R. At low DOC concentrations, GPP and R were nearly equal, but, at higher DOC, GPP and R uncoupled and lakes had negative NEE Strong correlations between lake metabolism and landscape related variables suggest that allochthonous carbon influences lake metabolism.
Article
We show that sediment respiration is one of the key factors contributing to the high CO2 supersaturation in and evasion from Finnish lakes, and evidently also over large areas in the boreal landscape, where the majority of the lakes are small and shallow. A subpopulation of 177 randomly selected lakes (<100 km2) and 32 lakes with the highest total phosphorus (Ptot) concentrations in the Nordic Lake Survey (NLS) data base were sampled during four seasons and at four depths. Patterns of CO2 concentrations plotted against depth and time demonstrate strong CO2 accumulation in hypolimnetic waters during the stratification periods. The relationship between O2 departure from the saturation and CO2 departure from the saturation was strong in the entire data set (r2=0.79, n=2 740, P<0.0001). CO2 concentrations were positively associated with lake trophic state and the proportion of agricultural land in the catchment. In contrast, CO2 concentrations negatively correlated with the peatland percentage indicating that either input of easily degraded organic matter and/or nutrient load from agricultural land enhance degradation. The average lake-area-weighted annual CO2 evasion based on our 177 randomly selected lakes and all Finnish lakes >100 km2 (Rantakari & Kortelainen, 2005) was 42 g C m−2 LA (lake area), approximately 20% of the average annual C accumulation in Finnish forest soils and tree biomass (covering 51% of the total area of Finland) in the 1990s. Extrapolating our estimate from Finland to all lakes of the boreal region suggests a total annual CO2 evasion of about 50 TgC, a value upto 40% of current estimates for lakes of the entire globe, emphasizing the role of small boreal lakes as conduits for transferring terrestrially fixed C into the atmosphere.
Article
Abstract Carbon dioxide gas flux across the air-water interface is most often treated as a ‘simple’ physical process, primarily responding to wind speed and water temperature. Available experimental data yield an exponential regression equation relating wind speed to the thickness of a stagnant boundary film through which gas diffuses to or from the water. Flux of CO2 is influenced by CO2 hydration reactions in the stagnant boundary layer. High pH and a thick stagnant boundary layer favour chemical enhancement of the CO2 gas flux. The rate of CO2 flux reflects the sum of net organic metabolism plus CaCO3 reactions. Some interesting gas-flux constraints on the rate of net organic carbon production and on global geochemical cycling of CaCO3 emerge. At high pH (circa 10), the maximum net organic carbon production which can be supported by CO2 flux across the air-water interface is about 0.06 mol C m&2 d&1. On a global scale, organic C, not atmospheric C, appears to account for the ‘CO2’ term in the classical CaCO3 dissolution-precipitation reaction.
Article
1. Using data from 71, mainly shallow (an average mean depth of 3 m), Danish lakes with contrasting total phosphorus concentrations (summer mean 0.02–1.0 mg P L−l), we describe how species richness, biodiversity and trophic structure change along a total phosphorus (TP) gradient divided into five TP classes (class 1–5: <0.05, 0.05–0.1, 0.1–0.2, 0.2–0.4,> 0.4 mg P L−1). 2. With increasing TP, a significant decline was observed in the species richness of zooplankton and submerged macrophytes, while for fish, phytoplankton and floating-leaved macrophytes, species richness was unimodally related to TP, all peaking at 0.1–0.4 mg P L−1. The Shannon–Wiener and the Hurlbert probability of inter-specific encounter (PIE) diversity indices showed significant unimodal relationships to TP for zooplankton, phytoplankton and fish. Mean depth also contributed positively to the relationship for rotifers, phytoplankton and fish. 3. At low nutrient concentrations, piscivorous fish (particularly perch, Perca fluviatilis) were abundant and the biomass ratio of piscivores to plankti-benthivorous cyprinids was high and the density of cyprinids low. Concurrently, the zooplankton was dominated by large-bodied forms and the biomass ratio of zooplankton to phytoplankton and the calculated grazing pressure on phytoplankton were high. Phytoplankton biomass was low and submerged macrophyte abundance high. 4. With increasing TP, a major shift occurred in trophic structure. Catches of cyprinids in multiple mesh size gill nets increased 10-fold from class 1 to class 5 and the weight ratio of piscivores to planktivores decreased from 0.6 in class 1 to 0.10–0.15 in classes 3–5. In addition, the mean body weight of dominant cyprinids (roach, Rutilus rutilus, and bream, Abramis brama) decreased two–threefold. Simultaneously, small cladocerans gradually became more important, and among copepods, a shift occurred from calanoid to cyclopoids. Mean body weight of cladocerans decreased from 5.1 μg in class 1 to 1.5 μg in class 5, and the biomass ratio of zooplankton to phytoplankton from 0.46 in class 1 to 0.08–0.15 in classes 3–5. Conversely, phytoplankton biomass and chlorophyll a increased 15-fold from class 1 to 5 and submerged macrophytes disappeared from most lakes. 5. The suggestion that fish have a significant structuring role in eutrophic lakes is supported by data from three lakes in which major changes in the abundance of planktivorous fish occurred following fish kill or fish manipulation. In these lakes, studied for 8 years, a reduction in planktivores resulted in a major increase in cladoceran mean size and in the biomass ratio of zooplankton to phytoplankton, while chlorophyll a declined substantially. In comparison, no significant changes were observed in 33 ‘control’ lakes studied during the same period.
Article
The Mackenzie River Delta, a floodplain system in the western Canadian Arctic contains approximately 45,000 lakes used by resident and migratory fish, aquatic birds, and mammal populations, which are critical resources for aboriginal peoples. Our results show the Mackenzie Delta is a remarkably productive aquatic ecosystem, not out of place relative to other large river floodplains and unusually productive for its Arctic latitude. Along with other such deltas of north flowing rivers to the Arctic Ocean, it represents a critical habitat of high productivity to support dependent consumers. Our results also showed a consistent under-saturation of CO2 in lake waters of the delta, co-occurring even with high concentrations of dissolved organic carbon, and indicating P:R greater than 1 during the open water period. In less turbid lakes, abundant macrophytes provide a considerable surface area for supporting production of epiphyton, but epiphyton production is constrained strongly by macrophyte shading, when macrophyte biomass is high. Despite this, epiphyton represented a greater supply of non-macrophyte algal carbon than phytoplankton, and thus explains why benthic algae may be a more important food source for primary consumers than phytoplankton, except in the most turbid systems. Most importantly, the high autotrophic production in the Delta lakes relative to nearby lakes on the arctic tundra suggests the extended ice-free season of the floodplain lakes and their landscape setting on, and replenishment by, nutrient-rich river sediments, is the strongest influence on aquatic production levels.
Article
We summarize rates of metabolism and major sources and sinks of organic carbon in the 148-k long, tidally influenced, freshwater Hudson River. The river is strongly heterotrophic, with respiration exceeding gross primary production (GPP). The P:R ration averages 0.57 (defined as the ratio of GPP to total ecosystem respiration) if only the aquatic portion of the ecosystem is considered and 0.70 if the emergent marshes are also included. Gross primary production (GPP) by photoplankton averages approximately 300 g C m−2 yr−1 and is an order of magnitude greater than that by submersed macrophytes. However, the river is deep, well mixed, and turbid, and phytoplankton spend a majority of their time in the dark. As a result, respiration by living phytoplankton is extremely high and net primary production (NPP) by phytoplankton is estimated to be only some 6% of GPP. NPP by phytoplankton and submersed macrophytes are roughly equal (approximately 20 g C m−2 yr−1 each) when averaged over the river. Emergent marshes are quite productive, but probably less than 16 g C m−2 yr−1 enters the aquatic portion of the ecosystem from these marshes. Heterotrophic respiration and secondary production in the river are driven primarily by allochthonous inputs of organic matter from terrestrial sources. Rates of metabolism vary along the river, with depth being a critical controlling factor. The P:R ratio for the aquatic portion of the ecosystem varies from 1 in the mid-river to 0.2 in the deeper waters. NPP is actually negative in the downstream waters where average depths are greater since phytoplankton respiration exceeds GPP there; the positive rates of NPP occurring upriver support a downstream advection of phytoplankton to the deeper waters where this C is largely respired away by the algae themselves. This autotrophic respiration contributes significantly to oxygen depletion in the deeper waters of the Hudson. The tidally influenced freshwater Hudson largely fits the patterns predicted by the river continuum model for larger rivers. However, we suggest that the continuum model needs to more clearly distinguish between GPP and NPP and should include the importance of autotrophic respiration by phytoplankton that are advected along a river. The organic carbon budget for the tidally influenced freshwater Hudson is balanced to within a few percent. Respiration (54%) and downstream advection into the saline estuary (41%) are the major losses of organic carbon from the ecosystem. Allochthonous inputs from nonpoint sources on land (61%) and GPP by phytoplankton (28%) are the major sources to the system. Agricultural erosion is the major source of allochthonous inputs. Since agricultural land use increased dramatically in the last century, and has fallen in this century, the carbon cycle of the tidally influenced freshwater Hudson River has probably changed markedly over time. Before human disturbance, the Hudson was probably a less heterotrophic system and may even have been autotrophic, with gross primary production exceeding ecosystem respiration.
Article
We present a Bayesian statistical model of diel oxygen dynamics in aquatic ecosystems to simultaneously estimate gross primary production, ecosystem respiration, and oxygen exchange with the atmosphere (and their uncertainties) on the basis of changes in dissolved oxygen concentration, water temperature, irradiance, and, if desired, the 18O to 16O ratio (d18O–O2). We test this model using simulated data with realistic measurement errors to demonstrate that it accurately estimates the model parameters and that parameter uncertainties correctly scale with error in the observations and number of data points. Application of the model to field data from two productive stream ecosystems with substantial daily dissolved oxygen variation quantified the underlying physical and biological factors that control oxygen dynamics in these ecosystems and provided empirical support for a light saturation model of the photosynthesis–irradiance relationships at the ecosystem scale. Although inclusion of d18O–O2 provides a second oxygen budget, analysis of field data shows that metabolic and reaeration parameters can be accurately estimated by modeling the transient dynamics of dissolved oxygen concentration alone in relation to daily changes in water temperature and light regime. This model is particularly suited to low– gas exchange, high-productivity systems, which have thus far proved challenging to measure ecosystem metabolism accurately. The modeling framework is applicable to single-station, open-system experimental designs and provides a rigorous and generalizable framework for estimating ecosystem metabolism in aquatic ecosystems.
Article
1. The biomass and production of picophytoplankton, large phytoplankton and heterotrophic bacterioplankton were measured in humic Lake Örträsket, northern Sweden during four consecutive summers. 2. High flow episodes, carrying fresh dissolved organic carbon (DOC) into the lake, always stimulated heterotrophic bacterial production at the expense of primary production. Primary production never exceeded bacterial production for approximately 20 days after such an episode had replenished epilimnial DOC. We suggest that allochthonous DOC is an energy source that stimulates bacterioplankton that, because of their efficient uptake of inorganic nutrients, are then able to outcompete phytoplankton. After the exhaustion of readily available DOC, phytoplankton were able to dominate epilimnion production in Lake Örträsket. 3. Biomass production was higher when dominated by phytoplankton than by bacterioplankton, despite a similar utilization of nutrients in the epilimnion throughout the summer. We propose that different C : N : P ratios of bacterioplankton and phytoplankton permit the latter to produce more carbon (C) biomass per unit of available inorganic nutrients than bacterioplankton.
Article
Cultural eutrophication has become the primary water quality issue for most of the freshwater and coastal marine ecosystems in the world. However, despite extensive research during the past four to five decades, many key questions in eutrophication science remain unanswered. Much is yet to be understood concerning the interactions that can occur between nutrients and ecosystem stability: whether they are stable or not, alternate states pose important complexities for the management of aquatic resources. Evidence is also mounting rapidly that nutrients strongly influence the fate and effects of other non-nutrient contaminants, including pathogens. In addition, it will be important to resolve ongoing debates about the optimal design of nutrient loading controls as a water quality management strategy for estuarine and coastal marine ecosystems.
Article
A quadratic dependence of gas exchange on wind speed is employed to analyze the relationship between gas transfer and wind speed with particular emphasizing variable and/or low wind speeds. The quadratic dependence is fit through gas-transfer velocities over the ocean determined by methods based on the natural C-14 disequilibrium and the bomb C-14 inventory. The variation in the CO2 levels is related to these mechanisms, but the results show that other causes play significant roles. A weaker dependence of gas transfer on wind is suggested for steady winds, and long-term averaged winds demonstrate a stronger dependence in the present model. The chemical enhancement of CO2 exchange is also shown to play a role by increasing CO2 fluxes at low wind speeds.
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